The invention relates to a vertical field type MRI apparatus for forming magnetic resonance images, including:
at least one field generating superconducting coil system for producing a substantially homogeneous magnetic field in an imaging volume of the apparatus,
which coil system includes:
a round outer coil that is situated in an outer coil plane;
a round supplementary coil that is situated within the outer coil.
An apparatus of this kind is known from U.S. Pat. No. 5,939,962. The homogeneous magnetic field required for MR imaging in such a vertical field type apparatus is usually generated by two oppositely situated magnetic poles wherebetween the patient to be examined can be arranged. Generally speaking, said magnetic field then has a vertical direction. Apparatus of this kind offers the advantage that the patient keeps a comparatively broad view of the surroundings when arranged in such an apparatus, so that sensations of claustrophobia occur less frequently.
An iron circuit that is capable of transporting the complete flux through the system becomes very heavy in the case of magnet systems having a field strength beyond approximately 0.5 T. A sensible alternative in that case is to omit the iron circuit completely and to construct the magnet system as an actively shielded air coil system. In that case there are no poles in the sense of iron structures that bound the space of the magnet system that is accessible to the patient, but the surfaces of the magnet system that bound the patient space will also be referred to hereinafter as xe2x80x9cpolesxe2x80x9d for the sake of simplicity. For field strengths beyond 0.5 T the coils must be constructed so as to be superconducting. They are kept at the operating temperature in a cryostat. The xe2x80x9cpolesxe2x80x9d are then formed by the outer wall of the vacuum envelope of the cryostat.
The cited United States patent discloses a superconducting coil system which consists of a round outer coil (a so-called xe2x80x9cside coilxe2x80x9d that is denoted by the reference 12a therein), a round supplementary coil (referred to therein as the xe2x80x9cfourth coilxe2x80x9d which bears the reference 12d), and a number of further coils (referred to as the xe2x80x9csecond and the third coilsxe2x80x9d bearing the references 12b and 12c therein). The homogeneous field in the imaging volume is generated mainly by the first two coils 12a and 12b and the other coils mentioned superpose a further homogenizing field thereon.
As is generally known, and also described in the cited U.S. patent, for this type of apparatus the aim is to arrange the field generating coil in the upper magnetic pole at an as small as possible distance from the field generating coil in the lower magnetic pole. This aim stems from the fact that the production costs of such a system increase by approximately a power of five of said pole distance, so that it is advantageous to keep this distance as small as possible. Because of this aim, the outer coils in the known apparatus are arranged practically directly against the boundary of the freely accessible space between the magnetic poles.
When the outer coils are mounted in that manner, the gradient coils in the known apparatus must extend to practically the diameter of the associated outer coils because of the necessary linearity of the gradient field in the imaging volume. Consequently, room for the supplementary coil (also having a voluminous and heavy construction so as to achieve the required homogeneous field) can be found only above the upper gradient coil and below the lower gradient coil. Consequently, the construction of this already bulky and heavy coil must be even larger; however, the outer coil must then also become larger. Moreover, in the case of actively shielded magnetic coils the shielding coils must then also become larger. The ultimate effect of the foregoing is that the costs of the apparatus are substantially increased again.
It is an object of the invention to provide an apparatus of the kind set forth in which the distance between the outer coils and between the supplementary coils is as small as possible. To achieve this, the apparatus in accordance with the invention is characterized in that
the energizing of the outer coil and of the supplementary coil is such that these coils generate magnetic fields of opposite direction,
the supplementary coil is also situated in the outer coil plane, and
the ratio Da/Do of the diameter Da of the supplementary coil to the diameter Do of the outer coil is between 0.7 and 0.9.
Because the supplementary coil is now also situated in the outer coil plane, the distance between the supplementary coils is minimized while taking into account the required dimensions of the imaging volume. A computer simulation of this configuration has demonstrated that no concessions need be made as regards the requirements in respect of field strength and/or homogeneity of the main field when use is made of said combination of said energizing and said diameter ratio. It has also been found that an adequate degree of freedom exists as regards the configuration of the further coils, that is, in dependence on the exact shape, dimensions and energizing of the outer coil and the supplementary coil.
An advantageous embodiment of the apparatus in accordance with the invention is provided with three further round coils. It has been found that a suitable compromise can thus be achieved between production costs (condition: few and small further coils) and field strength and homogeneity (condition: many and large further coils).
In a further advantageous embodiment in accordance with the invention the three further coils are situated on a conical surface, the apex of the conical surface being directed away from the imaging volume. Apart from the fact that this configuration very well satisfies the requirements as regards field strength and homogeneity, this embodiment notably offers the advantage that there is created an inner space (that is, a space around the vertical axis of the imaging volume) which, because of its conical shape, is very well compatible with a gradient coil having a conical external appearance. This shape of a cavity in the magnetic pole, that is, in the cryo container of the magnetic coils, also has an additional advantage. In normal operating conditions the cooling medium present in a cryo container, that is, liquid helium, has a pressure of approximately 1 bar. In given circumstances, however, this pressure may increase to as much as 3 bar. The helium container is surrounded by a vacuum space which, therefore, lies between the ambient atmosphere and the helium container. In the case of a cavity with square corners, as in the present state of the art, extreme mechanical stresses could occur at said pressures; when a more or less conical cavity is used, such stresses will occur to a much lesser extent because of the gradual shaping of the walls of the container.
A preferred embodiment of the MRI apparatus in accordance with the invention is provided with a second field generating superconducting coil system for producing the substantially homogeneous magnetic field in the imaging volume of the apparatus,
which second coil system includes:
a second round outer coil which is situated in a second outer coil plane and whose diameter is larger than that of the first outer coil,
a second round supplementary coil which is situated within the outer coil and in the outer coil plane,
the energizing of the second outer coil and of the second supplementary coil being such that these coils generate magnetic fields of opposite direction.
It is feasible to provide the coil system for generating the homogeneous field with only one pole surface; in such apparatus a given concession is made in respect of homogeneity and field strength, but it nevertheless remains possible to use the apparatus for given medical purposes. An apparatus of this kind is known, for example, from U.S. Pat. No. 5,917,395. When the apparatus is constructed so as to have two pole surfaces as is more usual, an attractive location of the imaging volume relative to the pole surfaces can be chosen. This offers an advantage in the following circumstances: a given size of the imaging volume is defined in dependence on the amount of space desired for the patient. This size defines the minimum distance between the pole surfaces. It should be possible to make optimum use of this distance for all imaging purposes, notably the imaging of parts of the body that are situated at a low level in the imaging volume, for example the vertebral column which is situated directly above the table top in the case of patients in the supine position. This table top, of course, should be constructed so as to be as thin as possible as otherwise space that could be used for imaging is lost or the pole surfaces have to be arranged further apart again. The vertebral column is then situated at the edge of the imaging volume, so that optimum homogeneity is not possible over a great length. As a result of said steps (notably because the lower outer coil has a diameter which is larger than that of the upper coil), the imaging volume can be lowered relative to the pole surfaces, so that the vertebral column fits better in the imaging volume without it being necessary to enlarge the latter (which is expensive and leads to a high power consumption during operation).
A further embodiment of the apparatus in accordance with the invention is provided with four further round coils, each of which is situated in a respective further coil plane, the outer coil plane being situated between the imaging volume and each of the further coil planes. It has been found that said number of further coils enables a suitable compromise to be achieved between production costs, field strength and homogeneity in the case of the desired lowering of the imaging volume.
In another embodiment yet of the invention the four further round coils in the apparatus are situated on a conical surface, the apex of the conical surface being directed away from the imaging volume. Thus, there is created a space for the main field within the coil container, the conical shape of said space being very compatible with a gradient coil having a conical external appearance.
Another embodiment of the apparatus in accordance with the invention is provided with at least a first gradient coil system and a second gradient coil system for producing a magnetic gradient field in the imaging volume of the apparatus,
each gradient coil system including a flat main gradient coil and a shielding coil, and
the first gradient coil system being situated in a space within the first field generating superconducting coil system and the second gradient coil system being situated in a space within the second field generating superconducting coil system. The gradient coils are thus suitably arranged in the space created by the invention in the container of the field coils for the homogeneous field.
At least one of the shielding coils in a further embodiment of the invention extends across a substantially conical surface whose apex is directed away from the imaging volume. It has been found that a gradient coil is thus formed which produces a gradient field of the appearance required for MRI, notably in respect of linearity. The external shape thus obtained is particularly suitable for accommodating the gradient coil in the cavity formed in the coil container in accordance with the invention.
Another embodiment of the apparatus in accordance with the invention is provided with a first and a second container for the first and the second field generating superconducting coil system, respectively, said containers being arranged to contain a cryogenic medium and communicating with one another in order to exchange the cryogenic medium, one of the containers being provided with a pressure connection for controlling the pressure in the containers as desired. In the case of a system of superconducting coils it may occur that a part of the coils does not to come into direct contact with the cooling medium (liquid helium), because a part thereof has evaporated and hence the coils situated at the highest level have lost said contact. Consequently, said coils may come out of the superconducting state; this is undesirable notably during operation. The entire system of coils, so the contents of both coil containers, can now be cooled by means of one helium system by making the lower container in this system serve also as a reservoir by storing more liquid helium therein than necessary for the lower coil holder alone. Should one of the coils in the upper coil container tend to loose contact with the liquid medium, the pressure in the lower container can be increased so that liquid is forced from the reservoir space to the upper container. The contact with the liquid is ensured without it being necessary to use separate level control for the relevant container.